Conventionally, examples of the ball spline devices of those types include one disclosed in JP 61-179414 U, one disclosed in JP 58-137616 A, and the like. Each of those ball spline devices includes a spline shaft provided with multiple ball rolling grooves extending in the axial direction thereof, and a nut member assembled to the spline shaft through an intermediation of a large number of balls and provided with endless circulation paths of the balls. In accordance with endless circulation of the balls, the nut member can freely move about the spline shaft along the longitudinal direction thereof.
Further, each of the endless circulation paths for the balls, which is provided to the nut member includes a load path in which the balls roll while bearing load acting between the nut member and the spline shaft, a ball returning path formed parallel to the loading path, and U-shaped direction converting paths for coupling the load path and the ball returning path with each other. Further, the direction converting paths are arranged at both ends of the load path and the ball returning path, whereby the endless circulation paths for the balls are formed.
When the ball returning path is formed by passing through the nut member itself, there is a tendency that the nut member is increased in thickness so as to be larger in diameter. In addition, when the diameter of the ball is small, the diameter of ball returning holes is also small, and hence it is difficult to form the ball returning holes directly to the nut member. Thus, in the ball spline devices disclosed in the above-mentioned documents, a retainer made of a synthetic resin is interposed in a gap between the inner peripheral surface of the nut member formed into a cylindrical shape and the spline shaft passing therethrough, and the ball returning paths and the direction converting paths are formed by cooperation of the retainer and the nut member.
Further, in order to move the balls rolling in the load paths to the ball returning paths through direction conversion, it is necessary to cause the balls to depart from the ball rolling grooves of the spline shaft. Therefore, in the conventional ball spline devices, ball scooping portions continuous with the direction converting paths are provided to the retainer. The balls having rolled in the ball rolling grooves climb up the ball scooping portions so as to be departed from the ball rolling grooves, and then accommodated in the direction converting paths formed by cooperation of the retainer and the nut member.    Patent Document 1: JP 61-179414 U    Patent Document 1: JP 58-137616 A